NAME

tmpfile — create a temporary file

SYNOPSIS

#include <stdio.h>

FILE *tmpfile(void);


DESCRIPTION

^[CX] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2024 defers to the ISO C standard.

The tmpfile() function shall create a temporary file and open a corresponding stream. The file shall be automatically deleted when all references to the file are closed. The file shall be opened as in fopen() for update (wb+), except that implementations may restrict the permissions, either by clearing the file mode bits or setting them to the value S_IRUSR | S_IWUSR.

^[CX] In some implementations, a permanent file may be left behind if the process calling tmpfile() is killed while it is processing a call to tmpfile().

An error message may be written to standard error if the stream cannot be opened.

RETURN VALUE

Upon successful completion, tmpfile() shall return a pointer to the stream of the file that is created. Otherwise, it shall return a null pointer ^[CX]  and set errno to indicate the error.

ERRORS

The tmpfile() function shall fail if:

[EINTR]

^[CX] A signal was caught during tmpfile().

[EMFILE]

^[CX] All file descriptors available to the process are currently open.

[EMFILE]

^[CX] {STREAM_MAX} streams are currently open in the calling process.

[ENFILE]

^[CX] The maximum allowable number of files is currently open in the system.

[ENOSPC]

^[CX] The directory or file system which would contain the new file cannot be expanded.

The tmpfile() function may fail if:

[EMFILE]

^[CX] {FOPEN_MAX} streams are currently open in the calling process.

[ENOMEM]

^[CX] Insufficient storage space is available.

EXAMPLES

Creating a Temporary File

The following example creates a temporary file for update, and returns a pointer to a stream for the created file in the fp variable.

#include <stdio.h>
...
FILE *fp;


fp = tmpfile ();

APPLICATION USAGE

It should be possible to open at least {TMP_MAX} temporary files during the lifetime of the program (this limit may be shared with tmpnam()) and there should be no limit on the number simultaneously open other than this limit and any limit on the number of open file descriptors or streams ({OPEN_MAX}, {FOPEN_MAX}, {STREAM_MAX}).

In multi-threaded applications, the tmpfile() function can leak file descriptors into child processes. Applications should instead use mkostemp() with the O_CLOEXEC or O_CLOFORK flag, or both, followed by fdopen(), to avoid the leak.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

2.5 Standard I/O Streams, fopen, mkdtemp, tmpnam, unlink

XBD <stdio.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

Large File Summit extensions are added.

The last two paragraphs of the DESCRIPTION were included as APPLICATION USAGE notes in previous issues.

Issue 6

Extensions beyond the ISO C standard are marked.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

• In the ERRORS section, the [EOVERFLOW] condition is added. This change is to support large files.

• The [EMFILE] optional error condition is added.

The APPLICATION USAGE section is added for alignment with the ISO/IEC 9899:1999 standard.

Issue 7

Austin Group Interpretation 1003.1-2001 #025 is applied, clarifying that implementations may restrict the permissions of the file created.

SD5-XBD-ERN-4 is applied, changing the definition of the [EMFILE] error.

SD5-XSH-ERN-149 is applied, adding the mandatory [EMFILE] error condition for {STREAM_MAX} streams open.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0668 [14] is applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0372 [678] is applied.

Issue 8

Austin Group Defects 411 and 1318 are applied, changing the APPLICATION USAGE section.

Austin Group Defect 1296 is applied, removing [EOVERFLOW] from the ERRORS section.